In a vehicle brake system for braking or stopping a vehicle, there is widely used a hydraulic brake system, namely, a brake system configured to restrain or stop rotation of wheels by a frictional force generated based on a pressure of a pressurized working fluid, to thereby brake or stop the vehicle. In recent years, there has been used, in hybrid vehicles and electric vehicles, for instance, a regenerative brake system, namely, a brake system in which a motor utilized for driving wheels is driven by rotation of the wheels and a resistance generated when the motor is driven is utilized for braking or stopping the vehicle while regenerating electricity. However, even in vehicles equipped with the regenerative brake system, it is sometimes difficult to obtain a sufficient braking force only by the regenerative brake system. Accordingly, in general, such vehicles are equipped with a brake system in which the hydraulic brake system and the regenerative brake system are combined, as disclosed in the following Patent Literature.